A magnetic field modulation magnetooptical recording/reproducing method has been known conventionally as a technique of making an optical disk highly dense.
As one example of conventional techniques, a consecutive light pulse irradiation and magnetic field modulation method described in JP-A-1-292603 and applied to an optical disk drive will be described. With this disk drive, clock signals are obtained from a preformatted clock pit train on an optical disk of a sample-servo format.
As shown in FIG. 8, while high output light pulses 802 synchronizing with clock signals 801 are irradiated, modulation magnetic fields 808 corresponding to data 803 are applied synchronously with the light pulses 802 to form magnetic domains 804. During the reproduction, the data 803 is detected by using the same clock signals 801. The characteristic feature of this method resides in that the edge distance 807 of the magnetic domain 805 recorded with too large a power is the same as the edge distance 807 of the magnetic domain 804 recorded with too small a power, irrespective of their different recording powers. It is therefore possible to record/reproduce always at a constant edge distance 807 and is suitable for high bit density recording/reproducing.
A second conventional example as a means for solving a recording medium sensitivity fluctuation problem associated with light modulation edge recording will be described with reference to JP-A-4-61028. According to the second conventional example, a recording medium is provided with a trial writing area at a predetermined position and a trial writing pattern is actually recorded in this trial writing area. By evaluating a signal reproduced from this trial area, optimization of a recording power level is performed.
FIG. 7 shows an example of the structure necessary for evaluating a reproduction signal for the optimization of recording conditions according to the second conventional example.
As shown in FIG. 7 at (a), a combination of two shortest/longest recording mark/gap repetition patterns determined from a recording modulation method is used as a trial writing pattern. If a (1,7) modulation method is used as a coding method, the lengths of shortest/longest recording mark/gap are 2 Tw and 8 Tw respectively (Tw is a channel bit length, i.e., a shortest change length of a recording mark, i.e., a detection window width). If the bit length of the recording code train is 0.53 microns, the longest mark/gap length is 3.0 microns. If the laser wavelength is 780 nm and the lens NA is 0.55, the amplitude of a signal reproduced from the repetition pattern (hereinafter called "coarsest pattern") of the longest recording mark/gap (each 8 Tw long) is generally determined only by the width of the recording mark, and the positions of leading and trailing edges of a signal correspond to the edge positions. On the other hand, the amplitude of a signal reproduced from the repetition pattern (hereinafter called "densest pattern") of the shortest recording mark/gap (each 2 Tw long) is smaller than the coarsest pattern because the recording mark/gap length is generally equal to a half the diameter of the reproduction light spot. The center level of the reproduction signal amplitude shifts toward the recording mark because of optical interference of the preceding and succeeding recording marks. This shift amount is influenced by both the length and width of the recording mark. The longer and wider the recording mark, the larger the shift amount. From the above consideration, the recording control has been performed so that the width of the recording mark becomes generally constant irrespective of the recording mark length, and the recording power level has been optimized by making the amplitude center level determined by the recording mark/gap (e.g., coarsest pattern) sufficiently longer than the diameter of the reproduction light spot become coincident with the reproduction signal center level of the densest pattern.
In the structure shown in FIG. 7 at (d) and disclosed in the above-cited publication, the center level of the amplitude of a signal reproduced from the densest/coarsest pattern is obtained as an average value of signal levels representative of the upper and lower envelopes. The peak and bottom levels of the reproduction signal 701 of the densest pattern are held by peak and bottom holding circuits 704 and 705, and the average level of the peak and bottom levels is held by a sample-hold circuit 707 by using a densest pattern detection gate 702 as a trigger. Similarly, the average level of a reproduction signal of the coarsest pattern is held by a sample-hold circuit 706 by using a coarsest pattern detection gate 703 as a trigger. A difference (V1-V2) between the two average levels is calculated by a differential amplifier circuit 708 to obtain a reproduction signal evaluation result signal 713 (.DELTA.V signal: .DELTA.V=V1-V2). The center level of a signal reproduced from the coarsest pattern changes scarcely even if the recording conditions shift more or less from the optimum conditions and recording mark/gap lengths are unbalanced more or less.
The structure shown in FIG. 7 at (e) is also disclosed in the above-cited publication as a method of evaluating the recording conditions from a reproduction signal. By using a low-pass filter 709 having a cut-off frequency lower than the frequency of a reproduction signal of the coarsest pattern, signal levels of the densest/coarsest patterns are sampled and held to form a reproduction signal evaluation result signal 714 (.DELTA.V signal). The recording power has been optimized by setting the recording power level so that .DELTA.V becomes 0. In this manner, recording can be performed always with generally a constant magnetic domain width.
FIG. 9 is a flow chart illustrating a basic sequence of the recording condition optimizing operation of the second conventional example. In this example, it is assumed that the recording medium is an optical disk and the trial writing pattern is a densest/coarsest pattern. When the recording condition optimizing operation starts, a trial writing area provided on a recording medium at a predetermined position is erased to prepare for the next writing operation. As the writing operation starts, a predetermined writing pattern is recorded on the recording medium under different recording conditions for each recording area (e.g., sector) which is the unit of recording management of the recording medium. After the recording operation is finished, the reproduction signal of each recording area is evaluated to determine the recording conditions most suitable for the optimum recording conditions. Since the recording conditions are different in the radial direction of the recording medium, the above recording operation is performed at proper radial positions of the recording medium (e.g., inner circumferential area, middle circumferential area, outer circumferential area, or each recording zone) to complete the trial writing operation.